Well logging method and apparatus utilizing periodically variable spontaneous potentials



2,550,005 a PERIODICA LLY April 1951 HENRI-GEORGES DOLL WELL LOGGINGMETHOD AND APPARATUS UTILIZIN VARIABLE SPONTANEOUS POTENTIALS FiledApril 22, 1948 3 n a 6 m 2 L W m 7 FIGJ.

A.C. ELECTRODE ELECTRODE PO TENTIA L POTENT/AL(RELAT/V) (RELATIVE)INVENTOR. HENRI-GEORGES DOLL HI ATTORNEYS.

Patented Apr. 24, 1951 WELL LOGGING METHOD AND APPARATUS UTILIZINGPERIODICALLY V A R I AB L E SPONTANEOUS POTENTIALS Henri-Georges Doll,Houston, Tex assignor to Schlumberger Well Surveying Corporation,Houston, Tex., a corporation of Delaware Application April 22, 1948,Serial N0. 22,616

6 Claims.

The present invention relates to well logging methods and apparatus.More specifically it relates to a novel method and apparatus utilizingalternating electro-filtration potentials for determining the levels ofthe more permeable formations traversed by a bore hole.

My prior Patent No. 2,433,746, discloses several different well loggingmethods and apparatus for locating permeable formations by means ofalternating electro-filtration potentials. In all of the systems theredisclosed, alternating electro-filtration potentials are produced bylower ing into a bore hole containing a column of liquid a mechanism forproducing localized, periodic fluctuations in the liquid pressure atdifferent depths. The location of the permeable formations isaccomplished by lowering an electrode into the bore hole atsubstantially the same level as the mechanism for producing the liquidpressure fluctuations, and obtaining indications of the alternatingpotentials picked up by the electrode.

The principal object of this invention is to provide a novel,alternating electro-filtration potential well logging method andapparatus in which periodic pressure pulses are produced in the borehole liquid without lowering special mechanism into the bore hole forthis purpose.

Another object of the invention is to provide a new and improved methodand apparatus of the above character in which alternatingelectrofiltration potentials are produced at the levels of permeableformations by applying a periodically varying pressure above the columnof liquid contained in the bore hole.

A further object of the invention is to provide a new and improved welllogging method and apparatus of the above character which requireslittle more than conventional equipment usually present at the well inthe field.

In accordance with the invention, the bore hole containing a column ofdrilling mud or other liquid is substantially completely closed off, andsuitable means is connected thereto at the surface for producingperiodic and uniform pressure pulses in the bore hole liquid. Theincrement of pressure contributed by the pressure pulse producing meansis large enough to insure the generation of alternatingelectro-filtration potentials of significant magnitude at the levels ofpermeable formations. The alternating potentials may be picked up by anelectrode moved through the well on a suitable cable, stuffing box meansbeing provided to facilitate movement of the cable through the wellwithout adversely 2 affecting the operation of the pressure pulseproducing means.

The invention may be better understood from the following detaileddescription of a typical embodiment, taken in conjunction with theaccompanying drawings in which:

Fig. 1 is a schematic diagram of alternating electro-filtrationpotential well logging apparatus constructed according to the invention;

Fig. 2 is a typical curve of variations in alternatingelectro-filtration potentials such as might be obtained in a welltraversing permeable formations; and

Fig. 3 is a typical curve of variations in alternatingelectro-filtration potentials superimposed on a standard spontaneouspotential curve.

The theory underlying the production of alterhating electro-filtrationpotentials is fully discussed in my above-mentioned prior Patent No.2,433,746 and it will not be necessary to repeat it herein. Sufficeit tosay that electro-filtration potentials can be produced in the vicinityof relatively permeable formations surrounding a bore hole containing acolumn of liquid by producing periodic pressure variations in the borehole liquid. For any given type of liquid and permeable formationmaterial, the electro-filtration potential produced is known to beproportional to the pressure differential at the boundary between thebore hole liquid and the formation and a given increment in pressureproduces a corresponding increment in the electro-filtration potentialobserved.

In Fig. 1 is shown a bore hole 10 drilled into the earth and traversinga plurality of subterranean formations such as relatively impermeableshales ll, i2 and I3 and relatively permeable formations M and i5.According to the usual practice, the upper part of the bore hole It maybe lined with a metallic casing I5 which extends to the surface of theearth. The bore hole !0 contains a column of liquid ll such as drillingmud, for example.

Connected to the casing is at the surface of the earth is the usual flowline l8 having a discharge valve is therein which is normally maintainedclosed. The casing is is also connected to a conduit 20 which receivesthe output of conventional pumping means 2i driven by any suitable primemover 22 in the conventional manner. The pumping means 2| is adapted topick up mud or other incompressible liquid 23 from the mud pit 24through an inlet 25 and to discharge it through the outlet 20 into thecasing iii.

For the proper practice of the present invention, it is essential thatthe bore hole In be substantially completely closed off. This may beaccomplished in any suitable manner as by securing a cap member 26 atthe top of the casing, for example. The cap member 26 may be providedwith a suitable packing gland or stuffing box 21 which is suitablydesigned to facilitate the passage of a cable 28 carrying an electrode29 into the well Ill. The stufling box 21 may be of any convenientconstruction and it may comprise, for example, two cooperating halves 3Dand 3| adapted to be fitted around the cable 28 to form a unitaryassembly which can be threaded or otherwise secured in the cap member26.

Formed within the stufling box 21 is a chamber 32 in the upper portionof which is disposed a snugly fitting packing member 33 designed toprevent any appreciable leakage of liquid around the cable 28. Thepacking member 33 may be of the split washer type and it should be madeof a suitable resilient material such as rubber, for example.

Mounted in the lower portion of the chamber 32 is a second packingmember 34, also made of resilient material such as rubber, for example.The packing member 34 may be provided with a bore 35 sufliciently largerthan the diameter of the cable 28 to permit a certain amount of leakageof liquid around the cable 28 into the chamber 32 for a purpose to bedescribed hereinafter. Fluid from the chamber 32 can flow through apassage 36 to an annular space 31 between the outside of the stufiingbox 21 and the inside of the cap member 26. The annular space 31communicates with an outlet 38 for discharging the liquid into the mudpit 24. A plurality of rollers 39 may be provided on the stufling box 21to guide the cable 28 and prevent excessive wear of the packing members33 and 34.

In operation, the bore hole In is completely filled up to the level ofthe stuffing box 2! by operation of the pumping means 2|, if necessary.The operation of the pumping means 2| is then continued with the flowline discharge valve l9 closed so that periodic, substantially uniformpressure pulses are produced in the bore hole liquid II. By virtue ofthe stuffing box construction described above, it will be understoodthat a certain amount of the bore hole liquid II will continuously leakthrough the space between the cable 28 and the bore 35 in the packingmember 34 into the chamber 32 in the Stuffing box 21.

Thus, each time the pumping means 2| produces a pressure increase in thebore hole liquid H, the leakage through bore 35 in the packing member 34causes the pressure to decrease to a reference value before the nextpressure pulse is received. As a result, periodic, substantially uniformpressure pulses are produced in the bore hole liquid I"! which result inthe production of alternating electro-filtration potentials in thevicinity of the more permeable formations such as the formations l4 andI5. As is known, these alternating electro-filtration potentials willhave the same frequency as the pressure pulses generated by the pumpingsystem described above.

The alternating electro-filtration potentials produced as describedabove may be picked up by the electrode 29 and transmitted through asuitably insulated conductor 40 in the cable 28 to the surface of theearth where they can be indicated or recorded in any suitable manner. Byconnecting the cable 40 to a recording instrument 4| having its otherterminal grounded at 42, a

curve of the type shown in Fig. 3 may be obtained. This curve is acomposite of the normal spontaneous potential curve (shown in dottedlines in Fig. 3) and the alternating electro-filtration potentials whichare superimposed thereon. As indicated in Fig. 3, the amplitude of thealternating component is substantially increased in the vicinity of thepermeable formations l4 and i5 so that the depths and thicknesses of thelatter can be readily determined.

If desired, indications may be obtained separately of the alternatingelectro-liltration potentials by connecting the conductor 43 in serieswith a D. C. blocking condenser 43 to a suitable recording instrument44, the other terminal of which may be grounded at 45 in the usualmanner. With this connection a curve of the type shown in Fig. 2 isobtained. In this figure, it will be noted that the amplitude of thealternating electro-filtration potentials is substantially increased inthe vicinity of the permeable formations l4 and I5 so that the depthsand thicknesses of the latter may be readily located from the logobtained.

In order to provide liquid pressure pulses of maximum amplitude for anygiven pump, the packing member 34 should be properly designed to permitsufiicient leakage of bore hole liquid through the bore 35 to restorethe liquid pressure within the top of the casing substantially toatmospheric pressure during each stroke. Obviously, the rate of leakageto provide maximum pressure pulses will depend upon the displacement ofthe pumping means 2| and the rate at which it is operated.

In practice, the packing member 34 should preferably be designed toprovide sufiicient leakage through the bore 35 to insure the productionof pressure pulses of maximum amplitude with the smallest pump that maybe expected in the field. Then, in cases where larger pumps areavailable, the additional leakage required, if any, may be provided bymanually adjusting the flow line discharge valve IE1, or by providinganother valve on the stuffing box.

In an actual test made in the field, pressure fluctuations from a levelof about 25 to 50 pounds per square inch to a level of about pounds persquare inch were obtained with a frequency of pulsation of the order ofone cycle in two seconds. Under these conditions, the levels ofpermeable formations traversed by the well could be readily detected bythe increase in amplitude of the alternating electro-filtrationpotentials obtained, the maximum amplitude being of the order of 5millivolts.

The above described method is of special utility in regions where thetype of formation is rather continuous but has relatively permeablezones at unknown locations. This condition often obtains in limestoneareas where the limestone is substantially impervious but has permeablesections of limited extent at different levels through which oil isproduced. In limestone areas of this type, the usual spontaneouspotential curves are sometimes difficult to interpret with the degree ofaccuracy required. However, curves of alternating electro-filtrationpotentials produced by pressure pulses in accordance with the presentinvention have been found very helpful in locating such producing zonesin limestone areas.

Generally speaking, the attenuation of the acoustic waves through themud column will increase with increases in frequency so it is usuallydesirable to use rather low frequencies such as a few cycles, or a fewtens of cycles, per second although the method is not limited to lowfrequencies. Also the bore hole liquid should preferably besubstantially incompressible. It will be apparent that the presence oflarge gas bubbles in the mud column might dampen the pressure pulses.Where a great quantity of undissolved gas is present, it is recommendedthat it be re-' moved. A conventional mud shaker or screen is usuallysufficient for this purpose.

It will be understood from the foregoing description, that the inventionprovides an effective method and apparatus for locating relativelypermeable formations traversed by a well. The

method and apparatus are especially suited for use in areas where theformations are relatively thick and continuous as in limestone areas,for example. Further, the greater part of the equipment required forpractising the method is usu ally present at the well in the field, andonly a small amount of additional equipment is required.

The specific embodiment described herein and illustrated in the drawingsis merely illustrative and can be modified considerably within thespirit of the invention. For example, other suitable means can be usedfor producing periodic pressure pulses in the bore liquid. In case astuffing box is used which does not permit any appreciable leakage ofthe bore hole liquid, a conventional pressure responsive valve designedto open when a predetermined fluid pressure is reached and to close whenthe pressure has dropped to a desired value may be mounted in the flowline IS, the discharge valve l 9 being kept open. Other modificationswill be apparent to those skilled in the art.

I claim:

1. In a method of logging the uncased portion of a well containing acolumn of conducting liquid and having an upper portion lined with acasing extending to the surface of the earth, the steps of lowering anelectrode on a supporting cable into the well, ailixing at the top ofthe well a cap having a stuifing box to facilitate the movement of thecable through the well, pumping a substantially incompressible liquidinto the well to increase the hydrostatic pressure in the well,permitting controlled leakage of liquid from the top of thewell topermit the increased hydrostatic pressure of the well liquid to decreasesubstantially to a reference level, repeating said liquid pumping stepwhile permitting controlled leakage from the well to produce periodicpressure pulses in the well liquid, the pulse repetition rate and therelative rates of pumping and leakage being suitably chosen to produceperiodically Variable electro-filtration potentials in the well at thelevels of relatively permeable formations, moving the electrode throughthe well during the production of said pressure pulses, and ob.- tainingindications only of periodically variable potentials picked up by said,electrode.

2. In a method for logging the uncased portion of a well containing acolumn of conducting liquid and having an upper portion lined with acasing extending to the surface of the earth, the steps of lowering anelectrode on a supporting cable into the well, afiixing at the top ofthe well a cap having a stuffing box to facilitate the movement of thecable through the well, pumping a substantially incompressible liquidinto the well to increase the hydrostatic pressure in the well,permitting controlled leakage of liquid from the top of the well topermit the increased hydrostatic pressure in the well to decreasesubstantially to a reference level, repeating said pumping step whilepermitting controlled leakage from the well to produce periodic pressurepulses in the well liquid, the pulse repetition rate and the relativerates of pumping and leakage being suitably chosen to produceperiodically variable electro-filtration potentials in the well at thelevels of relatively permeable formations, moving the electrode throughthe well during the production of said pressure pulses, and obtain ingindications of the total potential picked up by said electrode.

3. In apparatus for logging an uncased portion of a well containing acolumn of conducting liquid, the combination of an electrode movable inthe well on a cable extendin to the surface of the earth, well cap meanshaving stuiiing box means to facilitate passage of the cabletherethrough, means for periodically injecting into the top of said Wella substantially incompressible fluid under pressure so as toincrease thehydrostatic pressure in the well, means responsive to said increasedhydrostatic pressure for permitting controlled leakage of liquid fromthe well to release the pressure in said well gradually to a referencelevel after each injection of fluid therein, thereby producing periodicsubstantially uniform pressure pulses in the well liquid, the injectionrepetition rate and the relative rates of injection and leakage beingsuitably chosen to produce periodically variable electro-filtrationpotentials in the well at the levels of relatively permeable formations,and means for providing indications of periodically variable potentialspicked up by said electrode.

4. In apparatus for logging the uncased portion of a well containing acolumn of conducting liquid and having an upper portion lined with acasing extending to the surface of the earth, the combination of anelectrode movable in the well on a cable extendin to the surface of theearth, a cap member affixed at the top of said casing and having astuffing box to facilitate movement of the cable in the well, pumpingmeans for periodically injecting a substantially incompressible fluidunder pressure into the well to increase the hydrostatic pressuretherein, means responsive to said increased hydrostatic pressure forpermitting controlled leakage of liquid from the top of the well topermit the well liquid pressure to decrease substantially to a referencevalue after the creation of each pressure pulse, the injectionrepetition rate and the relative rates of injection and leakage beingsuitably chosen to produce periodically variably electro-filtrationpotentials in the well at the levels of relatively permeable formations,and means for providing indications of periodically variableelectro-filtration potentials picked up by said electrode.

5. In apparatus for logging the uncased portion of a well containing acolumn of conducting liquid and having an upper portion lined with acasing extending to the surface of the earth, the combination of anelectrode movable in the well on a cable extending to the surface of theearth, a cap member aflixed at the top of said casing and having astufiing box to: facilitate movement of the cable in the well, pumpingmeans for periodically injecting a substantially incompressible fluidunder pressure into the well to increase the hydrostatic pressuretherein, means in said stufiing box and responsive to said increasedpressure for causing controlled leakage around said cable of liquid fromthe well to permit the Well liquid pressure to decrease substantially toa reference value after the creation of each pressure pulse, theinjection repetition rate and the relative rates of injection andleakage being suitably chosen to produce periodically variable electro-filtration potentials in the well at the levels of relatively permeableformations, and means for providing indications of periodically variableelectro-filtration potentials picked up by said electrode.

6. In a method for logging an uncased portion of a well containing acolumn of conducting liquid, the steps of applying to the top of saidcolumn of liquid periodically varying pressure of such frequency andintensity as to produce periodically varying electro-filtrationpotentials in the well in the vicinity of relatively permeableformations traversed thereby, and obtaining indications of suchperiodically varying potentials as a function of depth in the well.

HENRI-GEORGES DOLL;

REFERENCES CITED lhe following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,167,066 Elliott July 25, 19392,190,686 Slichtel Feb. 20, 1940 2,304,051 Beers Dec. 1, 1942 2,317,039Ennis Apr. 29, 1943 2,347,615 Shelley Apr. 25, 1944 2,433,746 D011 Dec.30, 1947

